Method of forming a multi-layer paint film

A method of forming a multi-layer paint film comprising:

• forming a first base paint film in which an aqueous first base paint (A) is coated over an electrodeposited cured paint film;
• forming a second base paint film in which an aqueous second base paint (B) is coated over the first base paint film without carrying out preliminary drying by heating after forming the first base paint film;
• forming a clear paint film in which preliminary drying is carried out by heating after forming the second base paint film, a single-liquid type clear paint (C) is coated over the second base paint film;
• and
• heating and curing in which the first base paint film, the second base paint film, and the clear paint film are heated and cured at the same time, wherein the first base paint (A) includes water-soluble or water-dispersible polyurethane resin (A1) of acid value of less than 15 mg KOH/g and hydroxyl group value from 10 to 100 mg KOH/g and titanium oxide (A2) of specific surface area not more than 13 m2/g, and the water-soluble or water-dispersible polyurethane resin (A1) includes from 4 to 15 mass % with respect to the resin solid fraction of the water-soluble or water-dispersible polyurethane resin (A1) of at least one of ethylene oxide units and propylene oxide units, and the mass content ratio of the titanium oxide (A2) is from 40 to 60 mass % with respect to the paint solid fraction of the first base paint (A).